Frequently, bases are used in heat-developable photographic materials in order to promote development by heat. However, it is possible for development to occur beyond a level at which even if development time is further prolonged sensitivity does not come to increase though fog continues to increase owing to subtle variations in conditions such as temperature or time, resulting in a reduction in the image quality. In order to avoid this phenomenon in the diffusion transfer process, it has been proposed to use compounds which react with alkalies to release development stopping agents as described in U.S. Pat. No. 4,009,029, and acid polymers for neutralization as disclosed in Research Disclosure, Vol. 123, page 22 and Vol. 180, page 18030 and British Pat. No. 2,082,787A. In heat developable photographic materials, however, the former do not effectively stop development, and the latter cause a reduction in the density of the final image because the bases are rapidly neutralized.
The most effective development-stopping means conceivable is to perform development in the presence of a compound which releases an acid when even if development time is further prolonged sensitivity does not come to increase though fog continues to increase to neutralize the base which promotes development, thus stopping development. Very few compounds are known, however, which release acids when heated. For example, Japanese Patent Applications (OPI) Nos. 58642/74 and 57452/75 (corresponding to British Pat. No. 1,477,835) (the term "OPI", as used herein, refers to a "published, unexamined Japanese Patent Application") describe acid components which at a temperature of at least 60.degree. C. are dissolved, or release volatile acids. Since, the compounds disclosed in these patent applications neutralize the bases before heat development is started, development is inhibited and the density of the final images is reduced.